1. Field of the Invention
The present invention relates to data transmission apparatus and data transmission methods, and more particularly to serial data transmission of the computer capable of bus connection or serial connection to external devices.
2. Description of the Related Art
Conventionally, serial data transmission is known as one way to perform communication between DTE (data terminal equipment) and DCE (data circuit terminal equipment). Serial data transmission means transmission in which data is successively transmitted one bit at a time over a single data signal line to carry information from the sender side to the receiver side. Interfaces according to the CCITT (Consultative Committee for International Telephony and Telegraphy) X.21 standard are known as one type of interfaces between devices transmitting data by serial data transmission. In this type of interface, the DTE and the DCE are connected by five signal lines. More specifically, these five signal lines are a transmit data line (T line), a receive data line (R line), a control line (C line), an indication line (I line) and a signal element timing line (S line).
JP S60-12837A discloses a method for transmitting data in which, of the five signal lines connecting the DTE and the DCE, the T line, and the C line are combined to a T′ line, moreover the R line, the I line and the S line are combined to an R′ line, and data is transmitted with these two lines. With this data transmission method, the DTE and the DCE are provided, respectively, with a multiplexing means and a demultiplexing means, and both the DTE and the DCE are further provided with a means for extracting a synchronization signal from the signals that are multiplexed and demultiplexed by the multiplexing means and the demultiplexing means. Thus, data can be transmitted with fewer signal lines than conventionally.
However, with the above-described data transmission method, a clock signal for synchronizing the sending and receiving of data is transmitted together with the data signal, so that a device for increasing the frequency of the clock by a factor of 10 needs to be provided inside the DCE. Moreover, the above-described data transmission method is premised on bidirectional communication between the DTE and the DCE. Therefore, the data transmission efficiency for unidirectional data transmission from DTE to DCE is poor.